1. Field of the Invention
The present invention relates to a re-transmission controlling method applied to wireless transmission processing which executes re-transmission control and a wireless communication terminal which executes the re-transmission controlling processing, and to a technology suitable to be applied to data reception in a wireless communication terminal of the HSDPA method, for example.
2. Description of the Related Art
In the past, channel encoding (error-correction encoding), automatic repeat request (ARQ) and the like have been known as a technique to correct errors in wireless transmission applied to wireless telecommunication and so on. On the other hand, a technology called HARQ (Hybrid ARQ) in which an error-correcting function is incorporated has been developed, because packet errors caused by measurement errors, controlling delays or the like are inevitable when applying adaptive modulation/demodulation and error correction coding performed to improve throughput by selecting the optimum modulation method and encoding method depending on the state of a propagation path.
The HARQ is a re-transmission technique of performing in a physical layer a known ARQ-based re-transmission technology which has been provided in an upper layer of a wireless protocol and the like, in which an error-correcting function is combined to perform the re-transmission. Hence, the reliability of data supplied from a physical layer to an upper layer can be improved. In the ARQ that is a re-transmission technology of an upper layer, data which has not been received correctly is discarded and data to be re-transmitted is awaited in general, whereas in the HARQ, data which has not been received correctly is preserved as pre-decoding data of an error correction apparatus and is combined with re-transmitted data to be decoded. Combining data which has not been received correctly with re-transmitted data makes the likelihood showing the reliability of data improved, and increases the possibility of success in decoding.
Here, an example of wireless data communication to which the HARQ is applied is explained. As wireless data communication to which the HARQ is applied, High Speed Downlink Packet Access (hereinafter called HSDPA) method is, for example, proposed in which the high-speed downlink data transmission is executed in Universal Mobile Telecommunications System (UMTS system) that is a system to which W-CDMA is applied.
In the HSDPA method, protocol data units (hereinafter called PDU) correctly decoded using the HARQ function are rearranged in order of the transmission sequence number (hereinafter called TSN) included in header information of a PDU, and are transferred to an upper layer protocol. The HARQ processing and TSN rearrangement function before transferring data to the upper layer are executed in an MAC (Medium Access Control) layer. Further, protocol data units (PDUs) in the MAC layer are called MAC-PDUs. The MAC-PDUs include a MAC-hs PDU of a portion relating to the HSDPA method and a MAC-d PDU for a separate data transmission channel (DCH) existing in the W-CDMA system before introducing the HSDPA method, and the MAC-hs PDU includes a plurality of MAC-d PDUs.
Details of layer structure in the HSDPA method will be described in detail in an embodiment explained later on. In Non-patent Literature 1, communication standards in this HSDPA method are described in detail.
FIGS. 1A to 1E are diagrams showing relationships among protocol data units (PDUs), that is, among MAC-hs PDU, MAC-d PDU and RLC PDU in this HSDPA method. Transmission data supplied as shown in FIG. 1A is divided into data segments by a predetermined data amount as shown in FIG. 1B, and an RLC (radio link control) header is added to each data segment. The RLC header portion includes an SN (sequence number) value with which to rearrange RLC PDUs (RLC protocol data units) in order. Data to which the RLC header is added is sent as RLC PDU to a MAC-d layer and then a MAC header is added thereto as shown in FIG. 1C.
Data to which the MAC header added is sent as a MAC-d PDU (MAC-d protocol data unit) to a MAC-hs layer. Then a MAC-hs header is added by predetermined units as shown in FIG. 1D. Further, as shown in FIG. 1E, the resulted data is made into a MAC-hs PDU (MAC-hs protocol data unit), and is sent to a lower layer (transport channel layer) in which transmission processing is applied thereto. When receiving data, processing in the reverse direction to the flow from FIGS. 1A to 1E is executed to determine data units in each protocol from the data obtained from the transport channel layer, and the data shown in FIG. 1A is extracted.
FIGS. 2A to 2F are diagrams showing an example of communication states in accordance with the HARQ function proposed in the HSDPA method, which is a re-transmission technology incorporating an error-correcting function. FIG. 2A shows a transmission state of data packets (HS-PDSCH) at a base station, and FIG. 2B shows a reception state of data packets (HS-PDSCH) at the base station. The packet received here at the base station is either acknowledge signal (Ack signal) or negative acknowledge signal (Nack signals) from a terminal. The acknowledge signal (Ack signal) is a signal returned upon reception of a relevant packet having no data error on the side of a terminal, while the negative acknowledge signal (Nack signal) is a signal returned upon reception of a relevant packet having data errors.
A signal transmitted from the base station is received at a terminal as shown in FIG. 2C. The number shown in each packet in FIGS. 2A and 2C is a sequence number (TSN) as a number given to a packet. As shown in FIG. 2E, in a HARQ function unit in an MAC-hs layer, an error check (what is called a CRC check) using an error detecting code is performed on the data received at the terminal. When it is judged in this check that no error exists, an Ack signal is generated. On the contrary, when it is judged that there is an error, A Nack signal is generated.
Packets judged to have no error in the CRC check are sent to a reordering function unit in the MAC-hs layer as shown in FIG. 2F. Packets judged to have no error in the CRC check are discarded. Further, the Ack signals and Nack signals generated are transmitted from the terminal as shown in FIG. 2D and are received at the base station as shown in FIG. 2B.
Wireless transmission is executed as described above; and if, for example, packets of TSN=3, 4, 5 . . . are transmitted and an error is detected in the CRC check on the packet of TSN=5 as shown in FIG. 2E, a Nack signal is generated and is sent to the base station. When the Nack signal is detected in the base station and the same packet is ready to be re-transmitted, the same packet (packet of TSN=5 in this example) is re-transmitted as shown at the end on the right side in FIG. 2A. By the time the packet of TSN=5 ready to be re-transmitted, subsequent packets (packets of TSN=6, 7 in this example) to the packet of TSN=5 have been transmitted.
Therefore, on the terminal side, the packets of TSN=6, 7 subsequent to TSN=5 have been received before a correct packet of TSN=5 is again received, and rearrangement in the order of TSN is executed in the reordering function unit within the MAC-hs layer shown in FIG. 2F.
FIG. 3 is a diagram showing on a time line an example of processing in respective function units in a MAC-hs layer in related art, and the example in which a reception error has occurred to the packet of TSN=5 shown in FIG. 2. As shown in FIG. 3, a MAC-hs layer includes a HARQ function unit executing re-transmission controlling processing as described above and a reordering unit in which packets are reordered. Further, the MAC-hs layer shown in FIG. 3 includes a disassembly unit in which packets reordered in the correct order in the reordering unit are disassembled into data units to be dealt with in the MAC-d layer that is the next layer, and the data disassembled in the disassembly unit is sent to the MAC-d layer (at the top of FIG. 3). The disassembly processing in this disassembly unit is equivalent to the processing of disassembling the MAC-hs PDU shown in FIG. 1E into the MAC-d PDUs shown in FIG. 1D. At the time of this disassembly processing, the MAC-hs header shown in FIG. 1D is removed.
Processing in FIG. 3 is explained. As long as correctly received packets continue to be detected in the HARQ function unit shown at the bottom of FIG. 3, the reordering unit performs no processing of rearrangement and sends packets to the disassembly unit without rearranging. The disassembly unit disassembles each of the packets sent thereto, and then sends the results to the MAC-d layer which is the next (upper) layer. Specifically, the packets of TSN=3, 4 are, for example, directly sent to the disassembly unit to be disassembled, and then are sent to the MAC-d layer.
Then if the packet of TSN=5 is detected to have an error in the CRC check, received packets subsequent thereto are stored in a buffer within the reordering unit until the packet of TSN=5 is re-transmitted. Specifically, the MAC-hs PDUs of TSN=6, 7 are accumulated in memory areas secured by each timing of supplying a packet, and are temporarily stored. Then, upon re-transmitting the packet of TSN=5, the packets of TSN=6, 7 are read from the memory and are reordered in the correct packet order to be sent to the disassembly unit.
In the disassembly unit, after a delay of approximately an interval of the temporary storage in the memory, the disassembly processing is resumed starting from the packet of TSN=5, and the disassembled packets are sent to the MAC-d layer. With the processing as described above, correct reception data without an error is supplied to the MAC-d layer in the correct order.
[Non-patent Literature 1] 3GPP TS 25.321 Medium Access Control protocol specification (ver. 5.4.0 published in March 2003 3GPP)
As shown in FIG. 3, in the case where the TSNs of received packets stop continuing, the reordering unit within the MAC-hs layer accumulates received data in the buffer memory; and then when continuity is maintained using the data accumulated in the buffer, the received data accumulated are sent to an upper layer through the disassembly unit. Hence, in the case where this kind of processing is executed in a processor such as a CPU, a problem of a temporary high-load state occurs when a reception error occurs.
In particular, when continuity of received data becomes secured and data stored in the buffer are read, a large amount of data is sent to the disassembly unit at a time and processing of disassembling packets and processing of sending the results thereof to the MAC-d layer are intensively executed, with the result that a load to a processor constituting such layer temporarily becomes extremely high, which is a problem.
Further, in the HSDPA method, since the buffer used in the reordering unit is defined by the sum of the capacity thereof and a buffer capacity used in the RLC layer, it is desirable that memory actually used be dynamically secured. However, when securing the memory in a dynamic manner by means of a system call of a real-time OS or the like, since the amount of processing is relatively large, executing this kind of processing upon receiving each data will be considerable load on a processor.
With respect to this kind of high-speed packet transmission processing such as the HSDPA method, the amount of data (MAC-hs PDU size) dealt with at a time is extremely large, and therefore there is a possibility that throughput be deteriorated due to the reason of a transitional high load on a processor. Hence, it is important to prevent a transitional high-load state by dispersing a load to the processor as much as possible, however, countermeasures against that state have not been satisfactorily provided in the past.
Further, the amount of data temporarily stored in the buffer is also considerably large, so that reduction thereof is desirable.